Disruptions in ITER present the risk of generating large Runaway Electron (RE) beams that may cause untolerable damage when impacting plasma facing components. Assuming short Thermal Quenches (TQs) lasting around 1 ms, the hot tail mechanism, driven by the rapid temperature drop and the resulting non-Maxwellian electron population, may produce dangerous amounts of RE seeds [1]. This motivates modelling with 3D non-linear MHD codes to accurately account for electron losses due to magnetic field stochasticity and other 3D aspects of the dynamics. The related physics is highly difficult to capture and remains virtually unexplored. We leverage the JOREK 3D non-linear MHD code to post-process MHD simulations with test electrons. A dedicated computational framework has been developed to simulate a hot test electron population evolving in the pre-calculated MHD fields and undergoing collisions with the bulk electrons and ions. It has been verified in a reduced phase space against DREAM, and preliminary tested with 3D fields [2]. The new framework is employed to perform the first full-geometry estimates of mitigated ITER 15 MA H-mode scenarios, using MHD simulations presented in [4]. We observe that Shattered Pellet Injection (SPI) enables, for a degraded H-mode case, a safe disruption with a seed current below 10e-10 A mainly generated by a detrapping process. The successful mitigation seems due to the correct core dilution and the stochastic losses in the edge. However, the baseline H-mode case produces a potentially dangerous seed. Counter-intuitively, including the 3D effects increases the generated seed from 60 A to 670 A. For the first time, these simulations reveal a generation boost caused by the formation of acceleration cells due to the (1,1) helical cooling, as well as the transport of hot electrons from the core into these regions which overcome stochastic losses. [1]: Smith, H. M., & Verwichte, E. (2008). Hot tail runaway electron generation in tokamak disruptions. Physics of plasmas, 15(7). [2]: Puel, L., Nardon, E., Artola, F. J., Hu, D., & Jorek Team. (2025). Kinetic modeling of hot tail runaway electron generation during plasma disruptions using the JOREK code. Nuclear Fusion, 66(2), 026019. [3]: Paz-Soldan, C., and al. (2020). Runaway electron seed formation at reactor-relevant temperature. Nuclear Fusion, 60(5), 056020. [4]: Hu, D., Artola, F. J., Nardon, and al. (2024). Plasmoid drift and first wall heat deposition during ITER H-mode dual-SPIs in JOREK simulations. Nuclear Fusion, 64(8), 086005.